Objective
To investigate the role of histone H3 lysine 4 (H3K4) methylation in interleukin‐1β (IL‐1β)–induced cyclooxygenase 2 (COX‐2) and inducible nitric oxide synthase (iNOS) expression in human osteoarthritic (OA) chondrocytes.
Methods
Chondrocytes were stimulated with IL‐1, and the expression of iNOS and COX‐2 messenger RNA and proteins was evaluated by real‐time reverse transcriptase–polymerase chain reaction analysis and Western blotting, respectively. H3K4 methylation and the recruitment of the histone methyltransferases SET‐1A and MLL‐1 to the iNOS and COX‐2 promoters were evaluated using chromatin immunoprecipitation assays. The role of SET‐1A was further evaluated using the methyltransferase inhibitor 5′‐deoxy‐5′‐(methylthio)adenosine (MTA) and gene silencing experiments. SET‐1A level in cartilage was determined using immunohistochemistry.
Results
The induction of iNOS and COX‐2 expression by IL‐1 was associated with H3K4 di‐ and trimethylation at the iNOS and COX‐2 promoters. These changes were temporally correlated with the recruitment of the histone methyltransferase SET‐1A, suggesting an implication of SET‐1A in these modifications. Treatment with MTA inhibited IL‐1–induced H3K4 methylation as well as IL‐1–induced iNOS and COX‐2 expression. Similarly, SET‐1A gene silencing with small interfering RNA prevented IL‐1–induced H3K4 methylation at the iNOS and COX‐2 promoters as well as iNOS and COX‐2 expression. Finally, we showed that the level of SET‐1A expression was elevated in OA cartilage as compared with normal cartilage.
Conclusion
These results indicate that H3K4 methylation by SET‐1A contributes to IL‐1–induced iNOS and COX‐2 expression and suggest that this pathway could be a potential target for pharmacologic intervention in the treatment of OA and possibly other arthritic diseases.